In manufacturing semiconductor devices such as LSI and super-LSI or in manufacturing a liquid crystal display board or the like, a pattern is made by irradiating a light to a photosensitive resin coated on a semiconductor wafer or an exposure original plate for liquid crystal. However, if a dust adheres to the exposure original plate used in this pattern creating stage, the dust absorbs light or refracts it, causing deformation of a transferred pattern, roughened edges or black stains on a base, and leads to problems of damaged dimensions, poor quality, deformed appearance and the like. Incidentally, in the present specification, “exposure original plate” is a collective term of a mask for lithography (also simply referred to as “mask”) and a reticle. Now, we will explain this by taking a mask for example.
The above-mentioned lithography works are usually performed in a clean room, but, even in a clean room, it is still difficult to keep the exposure original plate clean all the time, therefore, the light irradiation is carried out only after a surface of the exposure original plate is sheltered by a pellicle.
In general, a pellicle is built up of a pellicle frame and a pellicle membrane, where the latter is attached to the former in a slack-free manner. The pellicle membrane is made of cellulose nitrate, cellulose acetate and a fluorine-containing polymer or the like which transmit well such lights that are used in light exposure (e.g., g-line, i-line, KrF excimer laser, and ArF laser). To attach the pellicle membrane to the pellicle frame, a solvent that dissolves the pellicle membrane well is applied to one of the two annular faces (hereinafter referred to as “upper annular face”) and, after pasting the pellicle membrane on it, the solvent and the membrane are dried by air flow, or alternatively an adhesive agent such as acrylic resin, epoxy resin and fluorine-containing resin is used to fix the pellicle membrane on the upper annular face of the pellicle frame.
Furthermore, on the other one of the two annular faces (hereinafter referred to as “lower annular face”) is laid a mask-bonding agglutinant layer made of a polybutene resin, a polyvinyl acetate resin, an acrylic resin and a silicone resin or the like for attaching the pellicle frame to the exposure original plate, and on this agglutinant layer is laid a releasable liner for protecting the agglutinant layer.
A pellicle is set in a manner such that the pellicle frame entirely surrounds the pattern region formed in the surface of the exposure original plate; and as the pellicle is installed for the purpose of preventing the dust from adhering to the exposure original plate, the pattern region is isolated from the external atmosphere by means of the pellicle so that the dust outside the pellicle cannot reach the pattern region.
In recent years, the design rule for LSI has been updated to demand resolutions in the order of sub quarter-micron, and in order to attain such higher resolutions the light sources having shorter wavelengths have come to be adopted. In practice, mercury lamp lights such as g-line (436 nm) and i-line (365 nm), which have been predominantly adopted as the exposure light source are being replaced by KrF excimer lasers (248 nm) and ArF excimer lasers (193 nm). As the density of the pattern is increased thus, the demand for higher flatness of the mask and silicon wafer is strengthened too.
After a completion of a mask, a pellicle is attached to the mask so as to prevent dust from attaching to the pattern of the mask, but as the pellicle is attached to the mask, there is a possibility that the flatness of the mask is changed. When the mask flatness is worsened it is possible that problems such as defocusing take place. Also, a change in the flatness of the completed mask may cause a change in the shape of the pattern printed on the mask, which would result in a problem of poor mask overlay alignment accuracy.
In recent years, with regard to the flatness required for a mask, the current requirement of a flatness of 2 micrometers across the pattern face is being replaced by more strict preferred values such as 0.5 micrometer or flatter for 65 nm node and beyond, and an updated preference is to require 0.25 micrometer or flatter.
In general, when a pellicle with a pellicle frame whose flatness is inferior to that of a mask is attached to the mask, the flatness of the mask changes owing a phenomenon of flatness/deformation transfer. When the flatness of the mask is changed, the pattern printed on the mask is deformed, and thus the pattern to be transferred by the exposure light shall deform too so that the pattern's positioning accuracy is degraded. In general, the light exposure in the semiconductor manufacturing process is repeated as many times as the number of the pattern layers, so that when the pattern's positioning accuracy is bad, a problem occurs that the alignment with respect to each layer becomes inaccurate. Also, nowadays studies are being made to adopt a double patterning wherein one pattern is divided into two masks and light exposure is carried out twice, one for each mask, to cope with the necessity of exposing more dense patterns to light. In such a case, an inaccurate positioning caused by pattern deformation directly affects the pattern dimension.
The pellicle is attached to a mask via the agglutinant layer provided on the lower annular face of the pellicle frame, and as the pellicle is attached to the mask, the pellicle is usually pressed against the mask with a force of about 20 to 40 kgf. In general, the flatness of the mask is a few micrometers or less, and is 1 micrometer or less for a state-of-the-art lithography mask, while the flatness of the pellicle frame is generally in the order of several tens of micrometers, which is larger compared with the mask. Because of this, when the pellicle is attached to the mask, the flatness of the mask sometimes changes due to the unevenness of the frame. Hence, it is possible to minimize the extent of the change in the flatness of the mask by increasing the flatness of the pellicle frame as high as that of the mask.
A pellicle frame is usually made of an aluminum alloy. With regard to a pellicle frame for semiconductor lithography, it is in general rectangular with a width of 150 mm or so and a length of 110 to 130 mm or so, and is constituted by an endless frame bar having a rectangular cross-section. The frame is generally prepared by punching out a rectangular hole through an aluminum alloy plate or by extrusion-molding an aluminum material, but as the width of the frame bar is as narrow as 2 mm or so, it is liable to deform, so to produce a frame of high flatness is not easy. Because of this, it is very difficult to make a pellicle frame, which has a flatness of the same degree as that of the mask.
Also, the Young's modulus of an agglutinant which is used to make the agglutinant layer to effect adhesion between the pellicle and the mask is normally about 1 MPa, which is far smaller than the Young's modulus 72 GPa or so of aluminum alloys of which a pellicle frame is usually made, so that the agglutinant layer is by far softer than the pellicle frame. For this reason, the agglutinant layer absorbs the unevenness of the surface of the pellicle frame, and as a result, it is thought, the influence of the unevenness of the frame upon the mask is mitigated. The Young's modulus of conventional agglutinants are 1 MPa or so, but if an agglutinant having a smaller Young's modulus value, that is, a softer agglutinant is used, it should be possible to increase the absorption of the unevenness of the pellicle frame into the agglutinant layer.
The softness of the agglutinant layer determines not only the amount of the absorption of the unevenness of the pellicle frame, but also the amount of the load added when the pellicle is attached to the mask. In use of a pellicle, a risk of creating a local air pass between the mask and the agglutinant layer as well as a serious risk such as detachment of the pellicle from the mask must be avoided. To this end, the agglutinant layer, which is inferior to the mask in flatness like the pellicle frame is, is made flatter by being pressed under a load so as to realize an air pass-free adhesion to the mask. In this connection, it is noted that the softer the agglutinant is, the smaller the load required will be to realize an air pass-free adhesion between the mask surface and the agglutinant surface. Namely, by using a softer agglutinant the force required for closely contacting mask surface and agglutinant surface becomes smaller, thus it is possible to attach the pellicle to the mask while keeping the deformation of the pellicle frame minimum.